Invasive Terrestrial Plant Field Guide: What Montana Prairie Field Evidence Can and Cannot Establish
How to read field-guide evidence for invasive land plants on the Montana prairie: which visible features support an identification, and where the record falls short.
Contents
The Short Answer
A field guide to invasive terrestrial plants is a reference for confirming what a plant is from features you can observe. It is not a guide to removing the plant, choosing garden plants or determining legal status. For the Montana prairie, the state's own Montana Field Guide is the appropriate regional reference for identification and occurrence. The material available for this article is limited to short excerpts and titles. Those excerpts support a few narrow statements, summarized below, but they do not provide enough detail to identify any invasive species to the species level. Treat everything here as a guide to how to read the evidence. Confirm any identification against the complete species account in an authoritative regional guide.
What the Available Sources Actually Say
Each source below addresses a different question, and none covers the whole topic. The table separates what each excerpt states from what it can and cannot support.
| Source | What the excerpt states | What it can establish | What it cannot establish |
| Montana Field Guide, invasive-species listing | Lists Common Blue-mustard (Chorispora tenella), with the other names Crossflower, Purple Mustard, Musk Mustard and Blue Mustard, and labels it a non-native species. Hydrilla appears in the same listing. | The state guide includes Common Blue-mustard in an invasive-species listing and classes it as non-native. | Its diagnostic features, its habitat, whether it occurs on any particular prairie site, or any legal designation. |
| A separately published terrestrial invasive plant field guide | For one species: flowers dark pink to purple, arranged in long flower spikes, with 1 to 15 flowering stems per plant. The guide credits this to the Invasive Species Centre. | A described combination of flower and stem features for one plant in that guide. | Which species is described, since the excerpt does not name it, or whether that guide's region includes Montana. |
| An ecological reference guide to plant invaders | States that its purpose is to give a scientific, ecological perspective on invasive plants. | That some references emphasize ecology alongside identification. | Prairie content. Its title focuses on forests and woodlands, which are different habitats from open prairie. |
| A Montana list pairing common invasive plants with native alternatives | Landscaping framing. | Nothing about field identification. | It addresses horticultural choice, which is outside the scope of a field guide. |
Two points about the state listing matter for anyone focused on land plants. First, the listing includes more than terrestrial plants. The guide maintains separate invasive-species groupings for organisms such as protists and bryophytes (mosses and their relatives). Second, the excerpt does not describe Hydrilla or its habitat. Before treating any species from a mixed listing as a terrestrial prairie plant, check its habitat in the full species account.
Which Visible Clues Help Identify an Invasive Plant
Field identification depends on combinations of features that are checked against a full species description. A single feature narrows the list of candidates, but it rarely settles the question. The one feature description in the available evidence shows how this works.
That description gives dark pink to purple flowers arranged in long flower spikes, with 1 to 15 flowering stems per plant. A spike is a type of inflorescence, meaning the arrangement of flowers on a stem, in which flowers are attached along a single elongated axis. Taken together, these features describe the overall look of the plant. They do not identify a species. Many unrelated plants have pink to purple flowers in elongated clusters. A count of 1 to 15 flowering stems is a wide range, and plenty of other plants fall within it. Because the excerpt never names the plant, these features cannot be tied to any species name in this article.
Clues That Are Insufficient on Their Own
- Flower color: it is easy to see, but it is shared across many plant groups and can be judged differently depending on light.
- Common names: Common Blue-mustard is also called Purple Mustard, Crossflower and Musk Mustard. A plant's names are labels, not field descriptions, and they should not be read as evidence of color, scent or structure.
- A place on an invasive-species list: being listed tells you that a species is considered non-native or invasive. It tells you nothing about what the plant looks like.
- Habitat setting: a prairie, roadside or disturbed patch gives useful context for an observation. It does not confirm identity, because both native and non-native plants can share the same setting.
What to Record for a Defensible Comparison
- Flower color, together with how the flowers are arranged on the stem, such as in a spike or in some other cluster.
- The number of flowering stems coming from one plant.
- Leaf arrangement and leaf shape along the stem.
- Fruit or seed structure, if it is present.
- Surrounding habitat, recorded as context rather than proof.
- Photographs taken from several angles, so the plant is documented without removing any material. Whether collecting is permitted is a separate question that this article does not address.
Compare these observations with the complete species account in a full regional field guide. That account should list the diagnostic features for the species and any native look-alikes. An identification is only as strong as its weakest link. If your observations match the features of a genus but not the features that separate one species from another, report the identification at the genus level.
Identification Is Separate From Status, Control and Garden Choice
Readers often blend four separate questions. Identification asks what the plant is. Status asks how a species is classified. "Non-native" means the plant occurs outside its native range. "Invasive" is an ecological judgment about how a species spreads and affects its surroundings. A noxious-weed designation is a legal classification made by a government body. Control and horticultural choice are management decisions. A field guide answers the first question. The evidence available here confirms only one status label, the non-native classification of Common Blue-mustard. It supports no conclusions about legal designations, removal methods, handling, edibility or replacement plantings.
Gaps in the Current Evidence
- No diagnostic features are available for Common Blue-mustard beyond its names and its non-native label.
- The plant with pink-to-purple flower spikes is unnamed in the excerpt, and that guide's geographic region is not established.
- None of the available material confirms where any invasive species occurs on the Montana prairie, or how common it is.
- No information on native look-alikes is available for any species.
- The ecological reference concerns forests and woodlands, not prairie.
- Full species accounts in the Montana Field Guide, or in another authoritative regional source, are needed before any of these gaps can be filled.
Related articles
Reading Soil Survey Maps for Prairie Nature Observation
How to read soil survey maps to predict prairie vegetation, wetlands and bird habitat around Medicine Lake, Montana, and how to plan careful, low-impact field visits.
Understanding Watershed Boundaries and Drainage Maps
What sets a watershed's boundary, how to trace one on a topographic map or in GIS, and why flat, pothole-dotted prairie like the country around Medicine Lake makes drainage maps harder to read.
Documenting Prairie Burrow Entrances and Occupant Uncertainty
How to record prairie burrow entrances, mounds and habitat in Montana grasslands while keeping the original digger, current occupant and unseen tunnels separate.
Reading Prairie Feeding Relationships and Their Evidence
How to tell observed consumption, apparent feeding, published diet links and ecological explanations apart when reading or building prairie food webs.
Documenting Fungi and Using Montana Identification References
How to photograph and record fungi on the Montana prairie, use Montana Field Guide PDF tools, read observation charts and judge mushroom ID apps.
Interpreting Prairie Land-Cover Classes and Map Uncertainty
How to read a prairie land-cover map's coverage, class definitions, resolution and accuracy figures before using it to plan grassland and wetland bird observation.
Match Montana Lichen Taxon Codes to References and Interpret Observation Charts
Match nine Montana Field Guide lichen codes to scientific names and synonyms, check their monthly observation charts, and learn what the counts can and cannot tell you.
Find Montana Amphibian Call References and Interpret Survey Records
Compare Montana amphibian call references, learn how the Natural Heritage Program's roadside listening protocol works, and tell a casual listening note apart from survey and monitoring evidence.
Compare Maps of Prairie Landforms and Sediments near Medicine Lake, Montana
How to compare the USGS Medicine Lake topo map, MBMG Report 738 and MBMG 540 to read glacial landforms and sediments while keeping buried geology separate.
Record Flower Features and Insect Visits Together
Petals are usually the colorful part of a flower that draws insects. Learn what to note about flowers and visitors in an informal, repeatable prairie field log.